1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method, wherein a soldering weight is used in, for example, a soldering step in manufacturing a power semiconductor module.
2. Description of the Background Art
In the process of manufacturing a power semiconductor module such as an IGBT module, the step of soldering power semiconductor elements to a circuit substrate, and the circuit substrate to a metal base plate is commonly carried out. When soldering a soldering article to a base material in this way, there may arise a problem such that when a solder interposed between the soldering article and base material melts, the position of the soldering article is shifted due to the surface tension of the molten solder, or the solder does not spread all over the bonding surface of the soldering article. As the method of suppressing this kind of problem, there is proposed a method of applying pressure to the molten solder by placing a weight on the soldering article when in the soldering step (for example, JP-A-2007-180456, JP-A-6-163612, and JP-A-2011-249578).
It is known that a warp occurs in the base material in the soldering step of the power semiconductor module.
For example, when soldering the base material and circuit substrate, the base material and circuit substrate have different coefficients of thermal expansion, meaning that the base material warps in an upward convex direction after solder has been solidified.
In the power semiconductor module, the warp of the base material in the upward convex direction is undesirable because it causes uneven spread in a coating of thermal compound when mounting the base material on heat radiating fins, thus leading to a decrease in heat radiation. Therefore, it is often the case that the base material, before being soldered, is intentionally warped in a downward convex direction in advance in order to cancel the warp caused when soldering and to flatten the base material. A schematic diagram is shown in FIG. 5.
In this way, it has been revealed from an earnest research by the present inventor that, when a soldering step is carried out with a weight 10 placed on a base material 30 having a warp when in the soldering step (FIG. 6A), as previously described, a solder 50 flows into the side with a lower height due to gravity when the solder 50 melts, meaning that the effect of evenly spreading the solder 50 over a bonding surface decreases even by mounting the weight 10. Because of this, the solder thickness of a portion of the base material 30 with a higher height decreases, and a void 51 is also generated (FIG. 6B), meaning that an adverse effect occurs in the heat radiation characteristics when the power semiconductor module operates.
Also, it has heretofore been common in manufacturing the power semiconductor module that a wiring step is carried out on the upper surface of the soldering article by wire bonding after the step of soldering the soldering article to the base material. In recent years, however, the step of carrying out an operation test in units of soldering articles after the wiring step has been carried out on the upper portion and then soldering only a good article to the base material is being prevailing. This is because the amount of loss is large when an operation test is carried out after all soldering articles have been soldered to the base material, and if only some of the soldering articles are found defective.
As the upper wiring interferes with the weight when soldering the soldering article, on the upper portion of which the wiring step has been carried out, to the base material in this way, it is not possible to use the technique of applying pressure to the molten solder using the weight in the previously described heretofore known technology.